The Science Case for a Titan Flagship-Class Orbiter with Probes
Total Page:16
File Type:pdf, Size:1020Kb
The Science Case for a Titan Flagship-class Orbiter with Probes Authors: Conor A. Nixon, NASA Goddard Space Flight Center, USA Planetary Systems Laboratory, 8800 Greenbelt Road, Greenbelt, MD 20771 (301) 286-6757 [email protected] James Abshire, University oF Maryland, USA Andrew Ashton, Woods Hole Oceanographic Institution, USA Jason W. Barnes, University oF Idaho, USA Nathalie Carrasco, Université Paris-Saclay, France, Mathieu Choukroun, Jet Propulsion Laboratory, Caltech, USA Athena Coustenis, Paris Observatory, CNRS, PSL, France Louis-Alexandre Couston, British Antarctic Survey, UK Niklas Edberg, Swedish Institute oF Space Physics, Sweden Alexander Gagnon, University oF Washington, USA Jason D. Hofgartner, Jet Propulsion Laboratory, Caltech, USA Luciano Iess, University oF Rome "La Sapienza", Italy Stéphane Le Mouélic, CNRS, University oF Nantes, France Rosaly Lopes, Jet Propulsion Laboratory, Caltech, USA Juan Lora, Yale University, USA Ralph D. Lorenz, Applied Physics Laboratory, Johns Hopkins University, USA Adrienn Luspay-Kuti, Applied Physics Laboratory, Johns Hopkins University USA, Michael Malaska, Jet Propulsion Laboratory, Caltech, USA Kathleen Mandt, Applied Physics Laboratory, Johns Hopkins University, USA Marco Mastrogiuseppe, University oF Rome "La Sapienza", Italy Erwan Mazarico, NASA Goddard Space Flight Center, USA Marc Neveu, University oF Maryland, USA Taylor Perron, Massachusetts Institute oF Technology, USA Jani Radebaugh, Brigham Young University, USA Sébastien Rodriguez, Université de Paris, IPGP, Diderot, France Farid Salama, NASA Ames Research Center, USA Ashley Schoenfeld, University oF CaliFornia Los Angeles, USA Jason M. Soderblom, Massachusetts Institute oF Technology, USA Anezina Solomonidou, European Space Agency/ESAC, Spain Darci Snowden, Central Washington University, USA Xioali Sun, NASA Goddard Space Flight Center, USA Nicholas Teanby, School oF Earth Sciences, University oF Bristol, UK Gabriel Tobie, Université de Nantes, France Melissa G. Trainer, NASA Goddard Space Flight Center, USA Orenthal J. Tucker, NASA Goddard Space Flight Center, USA Elizabeth P. Turtle, Johns Hopkins Applied Physics Laboratory, USA Sandrine Vinatier, Paris Observatory, CNRS, PSL, France Véronique Vuitton, Université Grenoble Alpes, France Xi Zhang, University oF CaliFornia Santa Cruz, USA Endorsed by: Veronica Allen, Universities Space Research Association, USA Carrie Anderson, NASA Goddard Space Flight Center, USA Shiblee Barua, Universities Space Research Association, USA J. Michael Battalio, Yale University, USA Patricia Beauchamp, Jet Propulsion Laboratory, CaliFornia Institute oF Technology, USA Ross A. Beyer, SETI Institute and NASA Ames Research Center, USA Julie Brisset, University oF Central Florida, USA John Cooper, NASA Goddard Space Flight Center, USA Daniel Cordier, CNRS, Université de Reims, France Martin Cordiner, Catholic University oF America, USA Thomas Cornet, Aurora Technology BV for ESA, ESAC, Spain Ellen C. Czaplinski, University oF Arkansas, USA Chloe Daudon, Université de Paris, IPGP, France Ravindra T. Desai, Imperial College London, UK Shawn Domagal-Goldman, NASA Goddard Space Flight Center, USA Chuanfei Dong, Princeton University, USA Sarah Fagents, University oF Hawaiʻi, USA Tom G Farr, NASA Jet Propulsion Laboratory, Caltech, USA William Farrell, NASA Goddard Space Flight Center, USA Lori K. Fenton, SETI Institute, USA Matthew Fillingim, University oF CaliFornia, Berkeley, USA Timothy A. Goudge, The University of Texas at Austin, USA Mark A Gurwell, Center For Astrophysics | Harvard & Smithsonian, USA Jennifer Hanley, Lowell Observatory, USA Tilak Hewagama, University oF Maryland, USA Amy E. Hofmann, NASA Jet Propulsion Laboratory, Caltech, USA Sarah Hörst, Johns Hopkins University, USA Der-You Kao, Universities Space Research Association, USA Mathieu Lapôtre, Stanford University, USA Sébastien Lebonnois, CNRS, Sorbonne Univ., France Liliana Lefticariu, Southern Illinois University, USA Alice Le Gall, LATMOS/IPSL, UVSQ, IUF, France Liming Li, University oF Houston, USA Nicholas A. Lombardo, Yale University, USA Chris McKay, NASA Ames Research Center, USA Delphine Nna-Mvondo, University oF Maryland Baltimore County, USA Alena Probst, NASA Jet Propulsion Laboratory, CalTech, USA Kerry Ramirez, Arizona State University, USA Miriam Rengel, Max-Planck-Institut Für Sonnensystemforschung, Germany Emilie Royer, Planetary Science Institute, USA Lauren Schurmeier, University oF Hawaiʻi, USA Edward Sittler, NASA Goddard Space Flight Center, USA Jennifer Stern, NASA Goddard Space Flight Center, USA Cyril Szopa, Université Paris-Saclay, France Alexander Thelen, Universities Space Research Association, USA Tuan H. Vu, NASA Jet Propulsion Laboratory, Caltech, USA 1.0 Introduction – The Need for a Titan Orbiter with Probes The joint NASA-ESA-ASI Cassini-Huygens mission [2, 3], which investigated the Saturnian system from 2004 to 2017, provided the first detailed look at its largest moon, Titan. Through 127 targeted flybys of the Cassini orbiter and in situ investigations of the lower atmosphere and surface by the Huygens probe, Titan was revealed for the first time. The mission uncovered surface features such as seas, lakes, dunes, mountains, filled and desiccated river valleys and plains [4-7] that were reminiscent of Earth in some respects but utterly different in others, as well as a dynamic atmosphere laden with organic molecules and replete with multiple layers of haze, clouds and rain [8-11] (Fig. 1). Titan provides a unique opportunity to study terrestrial processes in a completely different regime, and to learn about our home planet even as we learn about the solar system. Figure 1: Titan’s atmosphere and seas are in constant interaction, through aerosol sedimentation, precipitation, evaporation and other processes. A Flagship mission accomplishing remote sensing (orbiter) and in situ (probe) measurements, would build on Cassini’s legacy, and provide insights to key questions both For Titan, and our solar system as a whole. (Figure credit: [1]) In addition to its astounding successes, Cassini-Huygens left many questions about Titan unanswered (see Section 2). Insights into some of these questions will be provided by NASA’s forthcoming Dragonfly mission, which will land on Titan’s equatorial dune fields and sample the troposphere, surface and low latitude regions. However, Dragonfly will not address many other questions raised by Cassini-Huygens [12], including those related to global-scale geological history, atmospheric seasonal cycle and chemical processes, and origin and evolution of the polar seas, among other things. Furthermore, after Dragonfly only equatorial in situ measurements will 1 have been obtained. Therefore, a complementary mission to investigate Titan’s global-scale processes and make the first polar in situ measurements is highly desirable. These measurements could be accomplished by an instrumented orbiter, plus 1-2 entry probes that would investigate Titan’s polar seas, giving the first ground-truth data from the only seas aside from Earth’s found in the solar system, and a truly detailed global picture of Titan’s dynamic atmosphere and surface. Such a mission would advance NASA’s most current goals in planetary science and should be pursued in the coming decade. For example, it would “advance scientific knowledge of the origin and history of the solar system, the potential for life elsewhere” (NASA Science Plan, 2020), and improve our understanding of “how geologic processes on Mars and on ocean-bearing worlds in our solar system might give rise to habitable environments”. Similarly, it would address Strategic Objective 1.2: “scientific research of and from the Moon, lunar orbit, Mars, and beyond.” 2.0 Titan from Cassini-Huygens Through the Cassini-Huygens mission, Titan was discovered to have a subsurface water ocean, its surface was imaged in detail for the first time, and its atmospheric composition, thermal structure, and dynamics were examined from the surface to the exobase over two seasons. At low latitudes, Titan was found to be mostly dry, except for large episodic methane rain storms [9] (Fig. 2(a)). The Huygens landing site was found to potentially be a dried river bed, with (a) (b) (c) (d) Figure 2: Views oF Titan at low latitudes. (a) a vast arrow-shaped storm sweeps around Titan, wetting the surFace with methane rain (NASA/JPL/SSI); (b) branching river networks seen during the Huygens descent (NASA/ESA/University oF Arizona); (c) equatorial dunes showing east-west zonal morphology (NASA/JPL); (d) dunes Flowing around a crater (NASA/JPL). 2 evidence of past streamflow. However, no liquids were observed filling the channels that were seen incising nearby hills [8] (Fig. 2(b)). Indeed Titan’s methane hydrologic cycle is now the only other known example of such a cycle outside of Earth, composed of surface liquids, surface- atmosphere fluxes, atmospheric transport, and precipitation [13]. In addition, vast dune fields of apparently organic material girdle Titan’s equator, reaching hundreds of meters in height and hundreds of kilometers in length [5] (Fig. 2(c)). At mid-latitudes, Titan exhibits a mostly bland, undifferentiated appearance of as-of-yet undetermined organic material [14], perhaps overlaying and masking any historical record of impacts or geologic activity in the buried regolith. Impact (I) (II) Figure 3: Titan at high latitudes. (I) Northern seas (NASA/JPL/ASI); (II) Ontario Lacus near the south pole, showing changing shoreline From